Continuing studies by the prinicipal investigator and others have supported long-standing notions that information processing (or "use") in the developing nervous system may alter synaptic parameters. Light microscopic analysis of Golgi-stained brains indicates that rats reared in Complex Environments have more higher order dendrites on neurons in some cortical regions than rats from social or isolated rearing. Electron microscopic data indicates rats from complex environments have larger average synaptic contacts. Behaviorally, the complexity-reared rats are superior to isolation-reared counterparts on certain complex tasks. A series of experiments is in progress to further explore (a) the types of alterations in nervous systems after differential rearing, (b) their persistence into adulthood, (c) their reproducibility in adult animals, (d) their regional variations, and (e) their correlations with behavioral measures. It is proposed that these brain changes may represent a form of memory in the nervous system, in the sense that behavioral interactions with the environment affect later behavior. Whether more specific forms of memory may be similarly mediated is a topic for further study.